1. Field of the Invention
The present invention relates to a polishing apparatus and a polishing method and, more particularly, a polishing apparatus and a polishing method for use in planalization of insulating film, conductive film, etc. constituting a semiconductor device.
2. Description of the Prior Art
An integration density of semiconductor devices such as a semiconductor memory device is increasing with the years, and a multilayer structure of interconnection in their internal circuits is in progress more and more. Interlayer insulating films formed on the interconnection may be planalized by means of a Chemical Mechanical Polishing (referred to as "CMP" hereinafter) technique to permit such multilayer structure of interconnection. The CMP technique makes much account of end-point detection and automation of polishing in light of time and cost.
However, since polishing rate cannot be kept constant from various causes such as degradation of the abrasive cloth, it has been difficult to determine end of polishing strictly even if a polishing time is controlled. For this reason, up to now such working steps have been repeated until a planalized surface can be implemented that polishing is carried out for a short time and is then interrupted once to examine polished state of the object. The above working steps are not practical because these steps take much time and labor.
As a method of an end-point detection of the CMP, there has been up to this time a method wherein variation in torque of a motor for rotating a surface plate (to be also called as "head" hereinafter) is first detected and then frictional resistance of polished surface of the object is monitored based on the detected variation. But the method have little sensitivity since it can detect no high frequency component and thus it can detect only positional and time averages of a sliding friction caused between the polished surface of the object and the head. In addition, the method cannot be applied in some cases because of the structure of the head. For instance, in an air-back system in which the head and its enclosure are coupled to each other via an elastic material, it is difficult to transfer influence of friction caused by the polished surface to the rotation axis, so that detection sensitivity is extremely decreased. Therefore, the air-back system has not been adequate to practical use.
There has been another method in which the end-point of polishing is detected by measuring the polishing object through an optical thicknessmeter. But the end-point cannot be detected by the method in real time. Moreover, in case both a silicon nitride film and an SiO.sub.2 film have to be polished concurrently, a polished film thickness cannot be measured precisely through the optical thicknessmeter.
Hence, there have been proposed an end-point detection of polishing based on variations in a rotating torque of the motor and vibration of the surface plate in Patent Application Publications (KOKAIs) 6-320416 and 6-45299. However, in these Publications (KOKAIs), the end-point cannot be detected when the polished surface has been planalized simply, but it can be detected when different material is exposed to the polished surface with progress of polishing thereby to cause a change in frictional resistance of the polished surface and also a change in vibration.
There has been recited a method of measuring distortion of the surface plate due to friction between the polished surface and the abrasive cloth through a distortion sensor in Patent Application Publication (KOKAI) 6-320416.
However, since vibration caused by polishing is weak in a polishing apparatus in which a distortion sensor is put, mechanical vibrations (sound) such as motor vibration in the polishing apparatus are picked up by the distortion sensor as a background noise. Thus, sufficient sensitivity cannot be attained by the polishing apparatus. As a result, it become difficult to detect precisely polishing condition of the polished surface in an entire area or detect the end-point of polishing, and therefore additional polishing is required after fundamental polishing being completed.
If distortion of the head caused by friction between the polished surface and the abrasive cloth is to be measured by the distortion sensor, the distortion does not appear so obviously as to be picked up by the distortion sensor. Further, change in unevenness of the wafer surface cannot be detected by the distortion sensor in the actual circumstances even if vibration of the polishing apparatus itself is reduced by a filter, for the distortion sensor has no sensitivity to high vibration frequency.
In the conventional polishing apparatus, because no objective index concerning setting and replace timing of the abrasive cloth has been established, these workings are often executed wastefully.
In addition, if the polished surface has been scratched by dust (foreign materials) once in polishing, such scratch cannot be detected until the polishing object is taken out after the polishing to observe or check the polished surface through a microscope. No countermeasure has been taken against the dust which is produced and entered in the course of polishing the surface by virtue of the CMP. The polished surface has been merely estimated indirectly by observing scratches formed on the polished surface through the microscope.
Meanwhile, in the prior art, polishing condition has been estimated after completing the polishing. Therefore, even if the dust is entered into the polished surface in an initial stage of polishing the lot (usually 25 wafers) to begin scratching of the polished surface of the polishing object, mixture of the dust has not been able to be found until polishing of the concerned lot is terminated. For this reason, the objects to be polished after the dust acting to scratch the polished surface is entered have been scratched as a matter of course, so that the objects of polishing such as a semiconductor wafer are wasted. In addition, a part of the polished surface has been broken off as the dust from the scratched surface to thus increase the dust still more.
Moreover, location of the dust on the polished surface cannot be identified even if the dust exists in polishing the surface, and therefore in some cases the abrasive cloth must be exchanged as a whole to remove the dust. In such cases, it takes much time and labor to exchange the abrasive cloth.
Furthermore, in the foregoing Patent Application Publications (KOKAIs), to transmit the detection signal of the surface plate vibration from the surface plate to the amplifier has been recited. In the event that it is intended to transmit the signal via a radio system, the radio signal is interrupted temporarily by the shaft of the motor for rotating the surface plate.
In the meanwhile, an amount of polishing is dependent on profiles of patterns and in addition it changes widely according to polishing conditions such as applied pressure, number of revolution, flow rate of abrasive liquid, and surface conditions of the abrasive cloth. Accordingly, if an amount of polishing is controlled according to an elapsed time, trial-polishing would be carried out once lot by lot so as to confirm the polishing rate. However, a plenty of time and labor are required for such trial-polishing. Further, in the case that several kinds of lots including different patterns therein are polished, a time required for the trial-polishing is increased in the whole polishing time to thus reduce a throughput.